Downstream Fish Passage Design
Similar to upstream fish passage, there is no single solution for designing downstream fish passage facilities. Effective fish passage design for a specific site requires good communication between engineers and biologists and a thorough understanding of site characteristics.
Typical downstream fish passage at a hydroelectric dam occurs from the headpond to the tailrace through the powerhouse itself via the intake, penstock, and turbine or through a separate downstream passage system. For downstream passage through a hydroelectric powerhouse an estimate of downstream passage survival can be developed using industry-accepted and published turbine survival predictive algorithms and equations that factor in the fish’s body size and specific turbine characteristics. If turbine survival is too low, then a separate downstream passage system can be installed. Downstream passage facilities typically consist of two main components: a means to prevent entrainment into the powerhouse turbines and a bypass route to get fish safely to the tailrace. Prevention of entrainment at the powerhouse intake can be facilitated by installing close spaced trash racks; by installing a trash rack overlay during the specified downstream fish passage period; by installing an angled bar rack or louver array upstream of the intake to guide downstream migrating fish to the bypass route; or by installing a behavioral device such as an attraction flow pump to induce flow toward the bypass route. The bypass route for downstream migrants can consist of an existing waste gate or spillway gate, a notch in the existing spillway crest or spillway flashboards, or a new bypass pipe or open channel with an entrance adjacent to or through the intake trash racks. It should also be noted that the downstream end of any bypass route must discharge into a plunge pool of sufficient depth to prevent injury to the passing fish.
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